Lightpen control circuit

Abstract

A control circuit for a lightpen of a random scanning computer graphics sem (10) enabling the detection of lightpen interrupts or hits on contiguous vectors having drawing periods shorter than the response time of the lightpen (13). The lightpen interrupts are temporarily held in a peripheral circuit (31,32,34,36) and are selectively released to the computer system (via 29,21) at the end of its INTERPRET phase, i.e., at the end of a period during which the display processor decides how to handle the next display instruction. The present arrangement, which prevents the arrival of the lightpen interrupt signal during the INTERPRET phase, achieves a computer graphics system exhibiting maximum use of the information furnished by lightpen hits.

What is claimed is: 
     
       1. A light pen control circuit for releasing a computer interrupt signal in response to a light pen hit signal generated by a light pen during or after a display of graphical information by a random scanning computer graphics system which is capable of displaying vectors having a drawing period shorter than the response time of the light pen, comprising: means for generating a computer interrupt signal in response to a light pen hit signal generated within a predetermined interval corresponding to the display of graphical information or a prescribed period immediately thereafter;   means for determining whether the light pen hit signal occurred during or after the display of graphical information;   means for releasing the computer interrupt signal when the light pen hit signal occurred during the display of graphical information and for storing the computer interrupt signal when the light pen hit signal occurred after the display of graphical information; and   means for selectively releasing the stored computer interrupt signal contemporaneously with a display of subsequent graphical information.   
     
     
       2. The light pen control circuit according to claim 1 wherein said determining means comprises: means for generating a signal a prescribed period after the graphical information has been displayed; and   a logic gate for producing an output signal which changes state upon the simultaneous occurrence of said generating means signal and said light pen hit signal.   
     
     
       3. The light pen control circuit according to claim 2 wherein said storing and releasing means comprises: a flip-flop which is set in response to the output signal of said logic gate; and   means for releasing a signal from said flip-flop upon a subsequent display of graphical information of a prescribed nature.   
     
     
       4. A method for releasing computer interrupt signal in response to a light pen signal generated by a light pen during or after a display of graphical information which may comprise vectors having a shorter drawing period than the response of the light pen comprising the steps of: generating a computer interrupt signal in response to a light pen hit signal generated within an interval corresponding to the display of graphical information or a prescribed period immediately thereafter;   determining whether the light pen hit signal, which triggered the production of the computer interrupt signal, occurred before or after the display of graphical information;   releasing the computer interrupt signal when the light pen hit signal occurred during the display of graphical information and storing the computer interrupt signal when the light pen hit signal occurred after the display of graphical information; and   selectively releasing the stored computer interrupt signal contemporaneously with a display of subsequent graphical information.   
     
     
       5. The method according to claim 4 wherein said determining step comprises the steps of: generating the delay signal which terminates after a prescribed period following the display of the graphical information; and   generating a control signal which changes state upon the simultaneous occurrence of said delay signal and a light pen hit signal.   
     
     
       6. The method according to claim 5 wherein said selectively releasing step comprises the steps of: setting a flip-flop when said control signal changes states; and   releasing an output signal from said flip-flop upon the subsequent display of graphical information of a prescribed nature.